The present invention relates to an image recording apparatus, and more particularly to an image recording apparatus which expose a photographic film or the like to a light beam to record a highly defined image on the film in a predetermined range of image densities.
There have recently been developed radiation image recording and reproducing systems for producing he radiation image of an object using a stimulable phosphor material capable of emitting light upon exposure to stimulating rays (see, for example, Japanese Laid-Open Patent Publications Nos. 55-12429, 55-103472, 55-116340, 55-87970, etc). These radiation image recording and reproducing systems are finding wider use particularly in the medical field. When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
In the radiation image recording and reproducing system employing such a stimulable phosphor, the radiation image information of an object such as a human body is stored in a sheet having a layer of stimulable phosphor, and then the stimulable phosphor sheet is scanned wit stimulating rays such as a laser beam, so that the stimulable phosphor sheet emits light representative of the radiation image. The emitted light is then photoelectrically detected to produce an analog image signal which is then converted into a digital image signal representing digital image data. The digital image signal is thereafter electrically processed and converted again into an analog image signal. The analog image signal is used to modulate a laser beam, which is applied to a recording medium such as a photographic photosensitive material to record the image thereon. The recorded image is finally developed into a visible image.
It is known that if an image to be reproduced by the radiation image recording and reproducing system is a continuous-tone image, then it is expressed in a range of about 1,000 tones, or stated otherwise, the digital data of the image can be represented by 10 bits. Therefore, the digital-to-analog (D/A) converter, which is used to convert the digital image signal back into the analog image signal in the radiation image recording and reproducing system, may have a 12-bit resolution as long as its function of reproducing gradations is concerned.
However, the radiation image recording and reproducing system is actually required to correct images in view of film sensitivity characteristics, automatic developing machine characteristics, optical system transmittance characteristics, and laser power fluctuations. To allow for such image corrections, the D/A converter in the system should actually have a higher resolution of 14 through 16 bits.
Since high-resolution D/A converters have a long settling time, the time required for the system to process images is very long, and the number of images which can be processed by the system (or a cycle time) is limited. The 14- through 16-bit D/A converters are highly expensive, and as a result the radiation image recording and reproducing system which uses such a D/A converter is also costly to manufacture.